The present invention relates to a data recording method and, more particularly to an encoding method for use in a recording system in which a DC component is difficult to be reproduced, e.g., an encoding method for magnetically recording a data signal.
When a digital data signal is encoded, a precoding based on NRZI (non-return-to-zero inverted) conversion is carried out in order to avoid a propagation of error.
FIG. 1 of the accompanying drawings shows an example of such precoding based on NRZI conversion. As shown in FIG. 1, a digital data signal applied to an input terminal 21 is supplied to an adder (exclusive-OR circuit) 22 of which the output signal is supplied to an output terminal 23. The output from the adder 22 is also supplied to a 1-bit delay circuit 24, in which it is delayed by 1 bit and supplied to the adder 22. Therefore, an output signal shown on the lower row is obtained with respect to an input signal shown on the upper row of FIG. 2.
If the coding based on the NRZI is combined with a code conversion, such as an eight-to-nine conversion or the like, there is then obtained an encoded signal wherein the DC domponent of successive NRZI code words is minimized. Specifically, when the digital signal is encoded, there is obtained the encoded digital signal wherein a DSV (digital sum variation) obtained when +1 is assigned to symbol "1" and -1 is assigned to symbol "0" is constantly finite.
Since it is difficult to reproduce a DC component, when a coded signal is recorded on a magnetic medium, it is effective to obtain the encoded signal wherein the DC component of successive NRZI code words is minimized. In the NRZI conversion, if a code conversion such as an eight-to-nine (8/9) conversion or the like is combined with a preceding processing stage thereof, then there can be easily obtained the encoded signal wherein the DC component of successive NRZI code word is minimized. Therefore, the coded signal can be magnetically recorded satisfactorily.
U.S. Pat. No. 4,520,346 describes a technique in which a NRZI signal is converted to a DC-free signal.
In the encoding based on the NRZI conversion, it is possible to detect a code by using a partial response class 1. On the other hand, in the field of digital communication which is recently developed, a method for detecting a code using a partial response class 4 is used. Moreover, there has been established a detecting method with enhanced error correction ability, such as Viterbi decoding or the like.
When a code is detected by using the partial response class 4, precoding has to be carried out by I (interleaved)-NRZI conversion shown in FIG. 3.
As shown in FIG. 3, a digital data signal applied to an input terminal 41 is supplied through an adder (exclusive-OR circuit) 42 to an output terminal 43. An output signal of the adder 42 is supplied to 1-bit delay circuits 44, 45, in which it is delayed by 2 bits and supplied to the adder 42. Therefore, an output signal shown on the lower row in FIG. 4 is obtained with respect to an input signal shown on the upper row. Thus, a precoding based on the I-NRZI conversion is carried out.
In the I-NRZI conversion, a technique for converting a coded signal to a DC-free signal has not yet been realized. While the NRZI conversion can convert the coded signal to the DC-free signal with a cooperation of code conversion, such as eight-to-nine conversion provided at the preceding stage, the I-NRZI conversion cannot convert the coded signal to the DC free signal.
When a digital data signal is coded by a coding method based on the I-NRZI conversion in order to magnetically record a digital data signal, a DC component has to be suppressed by scrambling and randomizing or there has to be provided any means for reproducing a DC component. The above-mentioned method, however, needs extra circuit arrangements and cannot reproduce all codes.